Most people think mass incarceration in the U.S. is a recent thing, having started in the 80s with the war on drugs. This is a myth. Although the prison population does seem to suddenly shoot up at 1980, that’s just the way it is with exponential growth – only the most recent increases are noticeable. If we focus on the first 100 years of data, the sudden shoot-up seems to happen at 1920.

Furthermore, it was growing back then for the same reasons that it is growing now: changes in law.

This is what the scholars were saying back then:

“Characterizing the trends in imprisonment in the United States, Barnes and Teeters wrote, in 1959: ‘in 1931, 76 per cent of all the inmates of federal and state prisons had been incarcerated for committing acts that had not been crimes 15 years earlier.” The authors noted that, since 1900, 500,000 new state laws had been enacted in the United States, and they characterized this country as an “overcriminalized society,” with the recently enacted “victimless crimes” contributing significantly to the high rates of incarceration.

Trends in Incarceration in the United States since 1880: A Summary of Reported Rates and the Distribution of Offenses. Margaret Cahalan Crime Delinquency 1979 at 9 (emphasis added)

And this is what the scholars are saying now:

“While the U.S. has a higher rate of violent crime than many comparable nations, most scholars in the field attribute the dramatic increase in the use of prison almost entirely to changes in policy, and not crime rates. That is, policymakers at all levels of government have enacted laws and procedures designed to send more people to prison and to keep them in prison for longer periods of time.”

So, both the first half and the second half of the 20th century experienced an exponential rise in U.S. incarceration, and, in both cases, it was caused not by growing crime rates, but by new laws, much of which targeted drugs or other victimless crimes. It’s the same phenomenon, and it has the same explanation.